console.log('123' - '120');  //减号相减3
console.log(typeof('123' - '120')) //number
console.log('123'+0)  //加号变字符串直接拼接  123
console.log('123'-3)  //减号直接转换成数字运算 120
console.log('123'+'123')//加号拼接123123
console.log('0'+'0')  //00
console.log('0'-'1') //-1

//总结：只要是加号就是字符串，只要是减号就是数字